1. Field of the Invention
This invention pertains to microelectromechanical switches, and more particularly to the use of control circuitry to enhance performance and reliability of a switch.
2. Description of the Related Art
The following descriptions and examples are not admitted to be prior art by virtue of their inclusion within this section.
Micro-electromechanical switches, or switches made using micro-electro-mechanical systems (MEMS) technology, are of interest in part because of their potential for allowing integration of high-quality switches with circuits formed using integrated circuit (IC) technology. As compared to transistor switches formed with conventional IC technology, for example, MEMS contact switches may exhibit lower losses and a higher ratio of off-impedance to on-impedance. (“MEMS switch” and “micro-electromechanical switch” are used interchangeably herein, although the acronym does not correspond exactly.) The mechanical nature of a MEMS switch can create some performance problems, however. For example, the resistance of the switch when closed can be increased by aging or degradation of the switch contact surfaces, which can be caused by exposure to humidity and other contaminants. Such contamination can also lead to sticking of the switch and difficulty in opening it. Furthermore, the switching speed of a MEMS switch is generally lower than that of a transistor switch.
Addressing the above problems can be made difficult by tradeoffs inherent to MEMS switch operation. Modifications which improve closing performance of a switch, for example, may degrade its opening performance. In the case of a cantilever switch, for example, approaches to reducing the closing time of the switch include reducing the stiffness of the cantilever beam and reducing the gap between the contact element on the beam and the underlying contact pad. Unfortunately, these design changes typically have the effect of making opening of the switch more difficult. MEMS cantilever switch designs generally use an applied voltage to close the switch, and often rely on the spring force in the beam to open the switch when the applied voltage is removed. In opening the switch, the spring force, or restoring force, of the beam must typically counteract what is often called “stiction.” Stiction refers to various forces tending to make two surfaces stick together, such as van der Waals forces, surface tension caused by moisture between the surfaces, and/or bonding between the surfaces. In general, design modifications to a switch which act to reduce its closing time also tend to make the switch harder to open, such that the opening time may be increased, or the switch may not open reliably. It would therefore be desirable to develop ways to improve switch performance and reliability independent of the mechanical design of the switch itself.